1. Field of Invention
The present invention relates to the technical field of machining apparatus such as a machining center for machining a workpiece by means of a machining tool held by a holder removably fitted to a spindle. In particular, the invention relates to the technical field of machining apparatus that applies coolant to the machining tool and to the machined portion of a workpiece, with feed of the coolant during machining through coolant feed passages formed in the spindle, to the machining tool holder, and to the machining tool.
2. Description of Related Art
Conventionally, components of automatic transmissions (hereinafter also referred to as an “A/T”) are machined in various manners using a machining apparatus such as a machining center. In suc a machining the workpiece needs to be fixed on a table. For this purpose, the machining apparatus is provided with a clamping device for the workpiece.
The related art discloses a clamping device that clamps and unclamps a workpiece, actuated by means of an air cylinder (e.g., Japanese Patent Application Laid-Open No. HEI 11-156654 and Japanese Patent Application Laid-Open No. 2002-283168). The related art also discloses a clamping device that clamps and unclamps a workpiece, actuated by means of a hydraulic cylinder (e.g., Japanese Patent Application Laid-Open No. 2002-283168). In another machining apparatus of the related art a workpiece is clamped by actuating a clamp member by means of a gas sealed in a cylinder and that unclamps the workpiece by supplying the cylinder with a hydraulic pressure (e.g., Japanese Patent Application Laid-Open No. 2002-187037).
The related art also proposes to use a coolant as a machining liquid, a lubricant or a cooling agent during the machining of a workpiece by supplying the coolant at a predetermined pressure to a machined portion of the workpiece and that portion in contact with a machining tool, such as a drill held by a machining tool holder fitted to a spindle, via coolant supply passages formed in the spindle, the machining tool holder and the machining tool, and to return the used coolant to a coolant reservoir (e.g., Japanese Patent Application Laid-Open No. 2001-25939 or Japanese Patent Application Laid-Open No. 2001-287135).
The clamping devices disclosed in Japanese Patent Application Laid-Open No. HEI 11-156654, 2002-283168 and 2002-187037 use air pressure or hydraulic pressure to clamp and unclamp the workpiece and thus requires a pressure source. However, a dedicated pressure source for air pressure or hydraulic pressure creates problems such as complication of the apparatus, overall increase in the size of the machining apparatus, and increased cost. Moreover, a control system is required for clamping in addition to the control system for the machining. This complicates the overall control and further increases cost. In the clamp device disclosed in Japanese Patent Application Laid-Open No. 2002-187037, furthermore, since clamping force is obtained from pressure of a gas sealed in a cylinder, the clamping force changes with changes in gas pressure over time, differences in thickness of workpieces, and the like, resulting in additional problems. Thus, it is difficult to always clamp the workpieces with a constant clamping force.
On the other hand, in the machining apparatus disclosed in Japanese Patent Application Laid-Open Nos. 2001-25939 and 2001-287135, the coolant is used as a machining liquid, a lubricant or a cooling agent simply by supply at the predetermined pressure, to the machined portion of the workpiece where abutted by the machining tool during the machining of the workpiece.